Penetrating the mysteries of the Martian ice cap

Martian mysteries dating back four decades concerning the formation of a deep chasm and spiral troughs in the north polar ice cap have finally been solved thanks to new data from NASA's Mars Reconnaissance Orbiter, MRO.

Mars' north polar ice cap as seen by NASA's Mars Global Surveyor. It spans around 1,000 kilometres in diameter and is riddled with dark, spiral shaped troughs and a wide canyon (to the right of centre) that plunges to a depth of two kilometres. Image: NASA/JPL-Caltech/MSSS.

MRO's Shallow Radar instrument (SHARAD) uses radar waves to penetrate the subsurface. The reflected wave is sensitive to different types of material such as rock, sand, water and ice. Changes in the reflection characteristics of the radar wave also indicates layers of material.

“SHARAD is giving us a beautifully detailed view of ice deposits, whether at the poles or buried in mid-latitudes, as they changed on Mars over the last few million years,” says Rich Zurek, MRO project scientist.

Cross section of a portion of the north polar ice cap showing the internal ice structure. By studying the thickness, form and composition of the layers, planetary scientists can determine how each layer was deposited. Image: NASA/JPL-Caltech/ASI/UT.

At the north polar ice cap, which is a few kilometres thick, SHARAD has allowed scientists to reconstruct the large chasm known as Chasma Boreale and a series of spiral troughs, the formation of which has, until now, been much debated.

Chasma Boreale is around 500 kilometres long, 100 kilometres wide and up to 2 kilometres deep. Debate as to its formation has ranged from volcanic heat melting the bottom of the ice sheet and triggering flooding, to polar winds carving the canyon out of ice. The formation of the spiral troughs has been attributed to the spin of the planet – that is, ice closer to poles moves slower than ice farther away, causing cracking. In another model, increased solar heating in certain areas and lateral heat conduction could also generate troughs.

Now thanks to SHARAD, enough data has been collected to determine that both the canyon and the troughs have been shaped by a combination of material being deposited and then subsequently being moulded by winds over millions of years. By influencing the wind patterns, the shape of underlying, older ice controlled where and how the features evolved.

Top: Perspective view of northern polar cap of Mars, looking up Chasma Boreale. Yellow line indicates ground track of SHARAD in orbit. Bottom: Cutaway of Chasma Boreale showing subsurface layers as viewed by SHARAD. Detailed mapping of layers over many orbits reveals the history of ice accumulation and chasma formation. Image: NASA/Caltech/JPL/E. DeJong/J. Craig/M. Stetson

“Nobody realized that there would be such complex structures in the layers,” says Jack Holt of the University of Texas. “The layers record a history of ice accumulation, erosion and wind transport. From that, we can recover a history of climate that’s much more detailed than anybody expected.”

The study concludes that over the past 2.5 million years the troughs have migrated towards the poles as well as upwards in elevation by 600 metres. The data also shows that long-term processes such as the slow deposition of polar material, rather than catastrophic flooding for example, created the Chasma Boreale.

“These anomalous features have gone unexplained for 40 years because we have not been able to see what lies beneath the surface,” said Roberto Seu, Shallow Radar team leader at the University of Rome. “It is gratifying to me that with this new instrument we can finally explain them.”